In the first place, researchers consistently have found that mild punishment most often fails as a deterrent. Every parent has had the experience of giving a mild punishment most often fails as a deterrent. Every parent has had the experience of giving a mild punishment to a child, only to watch in exasperation as the child repeats the unacceptable behavior as if nothing had happened. Consider this example: When Laurie grabs her brother's toy out of his hands, her mother responds by slapping her on the wrist and recovering the toy for her brother. Laurie looks startled, but then proceeds to snatch the truck away once more, giving her mother a guilty little grin in passing.
With regards to spanking which most parents consider to be mild punishment, Dr. Murray Straus, a well-known researcher, conducted a study of 900 mothers of children between the ages of 6 and 9. He found that when the parents attempted to correct the child's behavior by spanking, the more they spanked the worse the child behaved two and four years later in spite of how much love, attention and affection the parents gave the child.*
*Excerpt from Dr. Thomas Gordon's F.E.T. Adult Resource Book